This invention relates generally to a miniature motor used in audio and video equipment, and automotive electrical components, and more particularly to a miniature motor which can be easily assembled, and has high resistance to vibration and impact, and high reliability in electrical connections.
FIG. 1 is a partially cross-sectional side view illustrating an example of a miniature motor to which this invention is applied. In FIG. 1, reference numeral 31 refers to a case made of a metallic material, such as mild steel, formed into a bottomed hollow tubular shape, and having an arc-segment-shaped permanent magnet 32, for example, fixedly fitted to the inner circumferential surface thereof. Numeral 33 refers to a case cover made of a thermoplastic resin material and formed in such a manner as to be engaged with an open end of the case 31. Numeral 34 refers to a rotor consisting of an armature 35 facing the permanent magnet 32 and a commutator 36, and rotatably supported by bearings 37 and 38 each provided on the case 31 and the case cover 33.
Numeral 39 refers to a brush arm made of an electrically conductive material, formed into a strip shape, having on a free end thereof a brush 40 which makes sliding contact with the commutator 36, and provided inside the case cover 33. Inside the case cover 33 also provided is an internal terminal 41 electrically connected to the brush arm 39 so that power can be fed to the armature 35 from an external d-c power source via the brush arms 39, brushes 40 and commutator 36.
Publicly known examples of this type of miniature motor include GB 2222730, GB 2244868, EP 511776, and GB 1594334.
With the aforementioned construction, as current is fed to the armature 35, rotating force is imparted to the armature 35 placed in a magnetic filed produced by the permanent magnet 32 fixedly fitted to the inner circumferential surface of the case 31, causing the rotor 34 to rotate, driving equipment connected to the rotor 34.
FIG. 2 is an internal end view showing an example of the case cover 33 in FIG. 1. Like parts are indicated by like numerals used in FIG. 1. In FIG. 2, the brush arm 39 is formed in an essentially L shape, fixedly fitted inside the case cover 33 by a holder 42 to force the brush 40 provided on a free end thereof onto the surface of the commutator (not shown) by the resiliency thereof. The internal terminal 41 is made of an electrically conductive strip material, with the contact area thereof making contact with an external power-feeding terminal 43 formed into an essentially S shape, and fitted inside the case cover 33 in such a manner as to cause the lower end thereof to make contact with the brush arm 39 to feed power to the brush arm 39.
FIG. 3 is an inside end view illustrating another example of the case cover 33 in FIG. 1. Like parts are indicated by like numerals used in FIG. 2 above. In FIG. 3, numeral 44 refers to a positive temperature coefficient resistor held in position and interposed between the essentially U-shaped brush arm 39 and the internal terminal 41. The positive temperature coefficient resistor 44 has such a characteristic that the internal resistance thereof sharply increases as the temperature exceeds a certain level (100.degree. C., for example). Consequently, if the temperature of the miniature motor is increased as a result of the continuous application of an overload to the miniature motor, or the flow of an overcurrent when the rotation of the rotor is forcibly locked, the internal resistance of the positive temperature coefficient resistor 44 sharply increases, rapidly reducing the value of current fed to the miniature motor to prevent the miniature motor from being overheated.
In the construction shown in FIGS. 2 and 3 above, the force pushing the brush arm 39 tends to be insufficient, lowering the reliability of electrical connection, because the elastic force imparted to the internal terminal 41 is relatively small. Furthermore, the work of assembling the internal terminals 41 into the case cover 33 is troublesome, resulting in low workability and productivity.
When inserting the external power feeding terminals 43 into the internal terminals 41, there is a possibility that the internal terminal 41 unwantedly moves in the direction orthogonally intersecting the end face of the case cover 33, depending on the method of inserting the external power feeding terminals 43. This could result in an interruption of power fed to the miniature motor, or lowered motor characteristics.
In the construction incorporating the positive temperature coefficient resistor 44, as shown in FIG. 3, pairs of the brush arms 39 and the internal terminals 41, which are the same components in terms of functionality, must be made in entirely different shapes and sizes for right-hand and left-hand ones because of the presence of the positive temperature coefficient resistor 44. This leads to a decrease in the number of common parts, and an increase in the number of parts, and therefore results in difficulty in parts control.